Watching the video we made helped me realize what we did wrong. First of all, we didn't expect to die. We expected a challenge, and maybe one or two dead members, but not the entire team.
Then, we didn't fully understand how the health potions work. They fill up when you kill enemies, not when you merely damage them, and in this boss fight there are no enemies to kill besides the boss. So no potion refills. We should have gone in with five medium-sized or bigger (if we could find them) health potions then, and nothing else in the potion belt.
I guess it's a good idea to record all the spots that we expect are going to give us trouble, both from a learning perspective, and from a showbiz one.